The present invention relates to the field of automatic transmissions for vehicles, and more particularly relates to a hydraulic pressure control system for controlling a transmission for a vehicle, said transmission having at least three speed stages, namely: a reduction geared speed stage, a directly connected speed stage, and an overdrive speed stage.
There are known various automatic transmissions for use in automotive vehicles. A typical such automatic transmission includes a gear transmission mechanism and a plurality of fluid pressure actuated friction engaging mechanisms. Selective supply of actuating fluid pressure to these friction engaging mechanisms provides various speed stages from the gear transmission mechanism. Further, various fluid pressure control systems are known for controlling such an automatic transmission, i.e., for providing supply of actuating fluid pressure to the friction engaging mechanisms. Such a fluid pressure system for an automatic transmission in general comprises: a source of fluid pressure which includes a fluid pump; a line fluid pressure control valve which modifies the fluid pressure supplied by the fluid pressure source and produces a line fluid pressure of a predetermined controlled pressure level; a throttle fluid pressure control valve, which receives supply of the line fluid pressure from the line fluid pressure control valve and which produces a throttle fluid pressure which increases in accordance with increase of the load on the engine of the vehicle, i.e., in the case of an internal combustion engine including a carburetor, in accordance with throttle opening (which is taken to be indicative of engine load); a governor fluid pressure control valve, which receives supply of line fluid pressure from the line fluid pressure control valve and which produces a governor fluid pressure which increases according to increase of the vehicle road speed; and a manual selector valve, which receives supply of the line fluid pressure from the line fluid pressure control valve, and which is controlled by the operator of the vehicle, by hand, so as to be selectively shifted between various shift ranges, such as D range, 2 range, etc.. Further, such a fluid pressure control system comprises a plurality of speed shift valves, which are shifted according to various equilibrium relationships between the governor fluid pressure and the throttle fluid pressure, so as selectively to supply actuating fluid pressures to the various friction engaging mechanisms. In such an automatic transmission, according to the driving conditions of the vehicle, i.e., according to the current values of vehicle road speed and engine load, the gear transmission mechanism is automatically shifted to the most desirable speed stage available therefrom, within the range of allowable gear shift stages which is currently manually selected by the vehicle operator on the manual selector valve.
It is general and conventional that one of the higher speed stages of such an automatic transmission should be a directly connected speed stage, i.e., a speed stage in which the gear transmission mechanism as a whole provides no increase or decrease of rotational speed. Further, such an automatic transmission also has at least one, and typically has two or three, reduction geared speed stages which are geared lower than the directly connected speed stage. These lower geared speed stages are used during acceleration of the vehicle from rest, during high acceleration at intermediate road speeds, during hill climbing or the like, of engine braking of the vehicle, and according to other particular combinations of operational conditions of the vehicle. It is also well known, and is becoming more and more frequent nowadays, for such an automatic transmission to be provided with an overdrive speed stage. When the gear transmission mechanism of the automatic transmission is set to such an overdrive speed stage, it provides an increase of rotational speed; in other words, its output shaft rotates faster than does its input shaft. This overdrive speed stage is typically the highest speed stage. As with the other speed stages, shifting of the gear transmission mechanism between the directly connected speed stage and the overdrive speed stage is controlled by a direct/overdrive shift valve, which is supplied with the throttle pressure and the governor pressure, and which shifts to and fro according to equilibrium relationships between the throttle pressure and the governor pressure. Thus, when vehicle road speed becomes so high in comparison with engine load that, according to such an equilibrium relationship, the governor pressure prevails over the throttle pressure, then, provided that the manual selector valve is selected by the vehicle operator to a range of speed stages which allows the engagement of overdrive speed stage (such as D range), then the direct/overdrive shift valve upshifts and selectively supplies actuating fluid pressure to the friction engaging mechanisms of the gear transmission mechanism so as to put the gear transmission mechanism into the overdrive speed stage; and, correspondingly, when engine load becomes so high in comparison with vehicle road speed that, according to such an equilibrium relationship, the throttle pressure prevails over the governor pressure, then the direct/overdrive shift valve downshifts and selectively supplies actuating fluid pressure to the friction engaging mechanisms of the gear transmission mechanism so as to put the gear transmission mechanism into the directly connected speed stage.
In the case of an automatic transmission, the gear transmission mechanism of which does not have an overdrive speed stage, and thus in which the directly connected speed stage is the highest gear stage, and the gear transmission mechanism of which further has at least one lower or reduction geared speed stage, then as an operational fact it is the case that, when the vehicle is being used in normal running conditions, the automatic transmission operates mostly in the directly coupled speed stage. In other words, if no overdrive speed stage is available from the automatic transmission, then, during usual operation of the vehicle, shifting between the directly connected speed stage and the reduction geared speed stage which is the next speed stage below the directly connected speed stage occurs rather infrequently. In fact, except during operational conditions involving deceleration of the vehicle either to the stationary state or to close to the stationary state, the operation of the vehicle is effected in a constant speed stage, i.e., the directly connected speed stage. Accordingly, wear and tear on the friction engaging mechanisms of the transmission is not very high, since the principal wearing of these friction engaging mechanisms occurs during shifting of speed stages.
On the other hand, in a conventional automatic transmission which is provided with an overdrive speed stage, the gear shift points between the overdrive speed stage and the directly coupled speed stage are conventionally located, in the shift diagram which shows the aforesaid balance relationships between throttle pressure and governor pressure as lines on a graph of engine load against vehicle speed, in a region which is frequently traversed by the point representing the actual operational conditions of the automobile. As a result, if the vehicle, as is usually the case, is not being driven at an absolutely constant road speed and engine load, frequent shifting between the directly coupled speed stage and the overdrive speed stage occurs. This causes problems and difficulties with regard to the durability of the automatic transmission, and with regard to drivability of the vehicle and the pleasure of operation thereof. In general, a hysteresis effect is provided for the direct/overdrive shift valve, i.e., in other words, at a given engine load, upshifting of this valve from the directly connected speed stage to the overdrive speed stage is arranged to occur at a somewhat higher vehicle road speed, than the speed at which downshifting of said valve from the overdrive speed stage to the directly connected speed stage is arranged to occur, as vehicle road speed decreases. However, in view of the conditions arising during actual operation of a vehicle, such a hysteresis as has been heretofore practiced has not solved the problem of instability of operation of an automatic transmission between the overdrive speed stage and the directly connected speed stage, and frequent changes of speed stage have still occurred during operation of the vehicle, accordingly causing quick wearing of the transmission, and poor durability of the automatic transmission and poor drivability of the vehicle in general.